Complete the DTD streaming validator with document-scoped ID/IDREF checks,
held in bounded sets (the identifier values, not the tree): a duplicate ID
is reported at its repeat occurrence, and forward IDREFs resolve at the end
of the stream via finish(). The streaming-vs-tree agreement test now covers
a duplicate ID as well. DTD streaming validation (content, attributes,
ID/IDREF) is now complete; RELAX NG and XSD streaming remain.
Part of #107, #105
Add PureXML.Validation.StreamingDTDValidator and PureXML.validate(streaming:
dtd:), validating a document against a DTD while it is pulled event by event
(the libxml2 xmlTextReader model), holding only the open-element stack rather
than the whole tree. Each element's content model and attributes are checked
as it closes by applying the exact per-element DTD rules to a shallow element
synthesized from the streamed child names, so streaming and tree validation
agree (a test asserts matching reasons). Errors carry an xmlGetNodePath-style
coding path. RELAX NG/XSD streaming and ID/IDREF streaming remain on #107.
Part of #107, #105
